1. Field of the Invention
The present invention relates to the detection of ionizing radiation. In particular, the present invention relates to a gamma radiation detector and a radioluminescent composition for use therein.
2. Discussion of Background
Many different types of radiation detectors are available, including ionization chambers, proportional counters, spark chambers, semiconductor detectors, scintillation detectors, and so forth. Ionization chambers and proportional counters are widely used due to their relative simplicity and stable performance. Solid state detectors and scintillation detectors are preferred for many applications because of their sensitivity and fast response.
Scintillation detectors use materials use materials that emit brief flashes of visible light when exposed to ionizing radiation. These materials are commonly known as scintillants, radioluminescent materials, or fluorescent or phosphorescent materials. The emitted light is in a wavelength region characteristics of the particular material, with intensity proportional to the energy absorbed by the material. The light is transmitted to a photosensitive detector such as a photomultiplier, converted to a current pulse, and amplified. A pulse height analyzer records the number of pulses per unit time (or counting rate) as a function of the amplitude of each pulse. the resulting spectrum reflects the energy distribution of the pulses.
The efficiency of a scintillant is proportional to its size and effective atomic number. Large, single crystals are needed to ensure that a large portion of the incoming energy is deposited in the detector. The higher the energy of the incoming radiation, the larger the crystal must be to detect the full energy event.
Both organic and inorganic scintillators are available. Organic scintillators are available in the form of pure single crystals, liquid solutions, or solid solutions (plastics). By way of example, Stuart, in U.S. Pat. No. 3,988,586, and Ambardanis, et al., in U.S. Pat. No. 3,931,523, disclosed radiation-detecting compounds incorporated into a plastic matrix.
The light emitted by an organic scintillator decays very rapidly, making possible detectors with time resolutions of less than 10.sup.-9 sec. Such detectors are used in liquid scintillation counting and as proton-recoil detectors in fast neutron time-of-flight studies. Organic scintillators have low effective atomic numbers and therefore exhibit low detection efficiency. While they are preferred for the detection of electrons, they are not effective gamma-ray detectors. In addition, plastic and other organic scintillators are not suitable for applications where exposure to environmental conditions might lead to degradation of the scintillator material.
Inorganic scintillators typically have higher atomic numbers and better energy resolution than organic scintillators. However, the light emitted by organic scintillators decays much more slowly than light from most organic scintillators, so their time resolution is poorer. Thallium-activated sodium iodide (NaI(Tl)) is widely used for gamma-ray detection, since it has a high atomic number, the highest light output of the room-temperature inorganic scintillators, and is readily available in large,, clear, single crystals up to 30 cm in size. The small admixture of thallium increases the light output and reduces the fluorescent decay time of the NaI crystal. NaI(Tl) is used in high-sensitivity, low-background gamma counting, large multiple-detector arrays, and high-efficiency gamma ray detectors. A disadvantage of NaI(Tl), especially for environmental monitoring or severe operating environments, is that the crystals are hygroscopic and require careful handling to avoid contamination.
Semiconductor detectors provide good energy resolution, but at efficiencies no more than about 30% of a NaI(Tl) detector due to their lower atomic number. Silicon is suitable for use in the relatively low energy X-ray range, while germanium is used in the higher energy X-ray and gamma-ray ranges. However, germanium detectors must be operated at cryogenic temperatures because of their high thermal noise levels and correspondingly high leakage currents. This renders them impractical for long-term monitoring, or use in areas where maintenance at cryogenic temperatures is impractical.
There is a need for an efficient radiation detector that is insensitive to environmental conditions including temperature and humidity. Materials having high atomic numbers offer the promise of higher detection efficiencies than the NaI(Tl) detectors presently considered the industry benchmark for gamma ray detection. Many naturally-occurring minerals, including anglesite (PbSO.sub.4) and cerussite (PbCO.sub.3) crystals, are known to fluoresce when exposed to short wave radiation. This property has been used to selectively detect and identify ores of potential economic interest. See, for example, Seigel, et al. (U.S. Pat. No. 4,365,15) and Fay (U.S. Pat. No. 4,336,459). Anglesite and cerussite also fluoresce when exposed to gamma radiation. Both minerals are found in the form of small, irregularly shaped crystals, unlike the large, uniform single crystals needed for an efficient detector. Growth of large single crystals of PbSO.sub.4 or PbCO.sub.3 has not been accomplished using present technology.